12 research outputs found
Alloy Formation at the Tetrapod Core/Arm Interface
The nature of the interfacial structure between the core
and the
arms of a tetrapod quantum dot (QD) formed during the heteroepitaxial
growth of a ZnS arm onto a CdSe core is not well understood but can
be analyzed through the use of high-frequency electron paramagnetic
resonance (HF-EPR) spectroscopy. The spectroscopic resolution at high
frequency allows the presence of unique crystal fields reflecting
interfacial alloying to be analyzed by incorporating MnÂ(II) ions as
a dopant into the QD to act as an intentional EPR active spectroscopic
probe. In addition, the HF-EPR can spectroscopically observe the presence
of ion vacancies that are anticipated to form at the heteroepitaxial
interface to accommodate structural mismatch. The HF-EPR spectra for
MnÂ(II) are extremely sensitive to perturbations of the microenvironment
due to changes in the crystal field. The HF-EPR spectra of MnÂ(II)
in a CdSe (core)/ZnS (arm) tetrapod exhibiting wurtzite symmetry for
both core and interface of the tetrapod provide clear evidence of
heteroalloying at the coreâarm interface and formation of intrinsic
dislocations at grain boundaries. The formation of the interfacial
alloy and grain boundaries reflects short-range ion migration at the
heteroepitaxial layer to reduce strain energy due to the 12% lattice
mismatch between the wurtzite lattices of CdSe and ZnS
Synthesis and Characterization of a Stable High-Valent Cobalt Carbene Complex
The formally Co<sup>IV</sup> carbene
CoÂ(OR)<sub>2</sub>(î»CPh<sub>2</sub>) is formed upon the reaction
of diphenylÂdiazoÂmethane
with the cobalt bisÂ(alkoxide) precursor CoÂ(OR)<sub>2</sub>(THF)<sub>2</sub>. Structural, spectroscopic, and theoretical studies demonstrate
that CoÂ(OR)<sub>2</sub>(î»CPh<sub>2</sub>) has significant high-valent
Co<sup>IV</sup>î»CPh<sub>2</sub> character with non-negligible
spin density on the carbene moiety
Synthesis and Characterization of a Stable High-Valent Cobalt Carbene Complex
The formally Co<sup>IV</sup> carbene
CoÂ(OR)<sub>2</sub>(î»CPh<sub>2</sub>) is formed upon the reaction
of diphenylÂdiazoÂmethane
with the cobalt bisÂ(alkoxide) precursor CoÂ(OR)<sub>2</sub>(THF)<sub>2</sub>. Structural, spectroscopic, and theoretical studies demonstrate
that CoÂ(OR)<sub>2</sub>(î»CPh<sub>2</sub>) has significant high-valent
Co<sup>IV</sup>î»CPh<sub>2</sub> character with non-negligible
spin density on the carbene moiety
High Field Electron Paramagnetic Resonance Characterization of Electronic and Structural Environments for Paramagnetic Metal Ions and Organic Free Radicals in Deepwater Horizon Oil Spill Tar Balls
In the first use of high-field electron
paramagnetic resonance
(EPR) spectroscopy to characterize paramagnetic metalâorganic
and free radical species from tar balls and weathered crude oil samples
from the Gulf of Mexico (collected after the Deepwater Horizon oil
spill) and an asphalt volcano sample collected off the coast of Santa
Barbara, CA, we are able to identify for the first time the various
paramagnetic species present in the native state of these samples
and understand their molecular structures and bonding. The two tar
ball and one asphalt volcano samples contain three distinct paramagnetic
species: (i) an organic free radical, (ii) a [VO]<sup>2+</sup> containing
porphyrin, and (iii) a Mn<sup>2+</sup> containing complex. The organic
free radical was found to have a disc-shaped or flat structure, based
on its axially symmetric spectrum. The characteristic spectral features
of the vanadyl species closely resemble those of pure vanadyl porphyrin;
hence, its nuclear framework around the vanadyl ion must be similar
to that of vanadyl octaethyl porphyrin (VOOEP). The Mn<sup>2+</sup> ion, essentially undetected by low-field EPR, yields a high-field
EPR spectrum with well-resolved hyperfine features devoid of zero-field
splitting, characteristic of tetrahedral or octahedral MnâO
bonding. Although the lower-field EPR signals from the organic free
radicals in fossil fuel samples have been investigated over the last
5 decades, the observed signal was featureless. In contrast, high-field
EPR (up to 240 GHz) reveals that the species is a disc-shaped hydrocarbon
molecule in which the unpaired electron is extensively delocalized.
We envisage that the measured <i>g</i>-value components
will serve as a sensitive basis for electronic structure calculations.
High-field electron nuclear double resonance experiments should provide
an accurate picture of the spin density distribution for both the
vanadyl-porphyrin and Mn<sup>2+</sup> complexes, as well as the organic
free radical, and will be the focus of follow-up studies
Spin Dynamics and Relaxation in Graphene Nanoribbons: Electron Spin Resonance Probing
Here we report the results of a multifrequency (âŒ9, 20, 34, 239.2, and 336 GHz) variable-temperature continuous wave (cw) and X-band (âŒ9 GHz) pulse electron spin resonance (ESR) measurement performed at cryogenic temperatures on potassium split graphene nanoribbons (GNRs). Important experimental findings include the following: (a) The multifrequency cw ESR data infer the presence of only carbon-related paramagnetic nonbonding states, at any measured temperature, with the <i>g</i> value independent of microwave frequency and temperature. (b) A linear broadening of the ESR signal as a function of microwave frequency is noticed. The observed linear frequency dependence of ESR signal width points to a distribution of <i>g</i> factors causing the non-Lorentzian line shape, and the <i>g</i> broadening contribution is found to be very small. (c) The ESR process is found to be characterized by slow and fast components, whose temperature dependences could be well described by a tunneling level state model. This work not only could help in advancing the present fundamental understanding on the edge spin (or magnetic)-based properties of GNRs but also pave the way to GNR-based spin devices
Evidence of a ZnCr<sub>2</sub>Se<sub>4</sub> Spinel Inclusion at the Core of a Cr-Doped ZnSe Quantum Dot
Herein we report doping of ZnSe by Cr ions leads to formation
of
small ZnCr<sub>2</sub>Se<sub>4</sub> spinel inclusions within the
cubic sphalerite lattice of a 2.8 nm CrZnSe quantum dot (QD). The
Cr ion incorporates as a pair of CrÂ(III) ions occupying edge-sharing
tetragonal distorted octahedral sites generated by formation of three
Zn ion vacancies in the sphalerite lattice in order to charge compensate
the QD. The site is analogous to the formation of a subunit of the
ZnCr<sub>2</sub>Se<sub>4</sub> spinel phase known to form as inclusions
during peritectoid crystal growth in the ternary CrZnSe solid-state
compound. The oxidation state and site symmetry of the Cr ion is confirmed
by X-ray absorption near edge spectroscopy (XANES), crystal field
absorption spectroscopy, and electron paramagnetic resonance (EPR).
Incorporation as the CrÂ(III) oxidation state is consistent with the
thermodynamic preference for Cr to occupy an octahedral site within
a IIâVI semiconductor lattice with a half-filled t<sub>2g</sub> d-level. The measured crystal field splitting energy for the CrZnSe
QD is 2.08 eV (2.07 eV form XANES), consistent with a spinel inclusion.
Further evidence of a spinel inclusion is provided by analysis of
the magnetic data, where antiferromagnetic (AFM) exchange, a CurieâWeiss
(CâW) temperature of Ξ = â125 K, and a nearest-neighbor
exchange coupling constant of <i>J</i><sub>NN</sub> = â12.5
K are observed. The formation of stable spinel inclusions in a QD
has not been previously reported
Enhancing the Magnetic Anisotropy of Linear Cr(II) Chain Compounds Using Heavy Metal Substitutions
Magnetic properties of the series
of three linear, trimetallic chain compounds Cr<sub>2</sub>CrÂ(dpa)<sub>4</sub>Cl<sub>2</sub>, <b>1</b>, Mo<sub>2</sub>CrÂ(dpa)<sub>4</sub>Cl<sub>2</sub>, <b>2</b>, and W<sub>2</sub>CrÂ(dpa)<sub>4</sub>Cl<sub>2</sub>, <b>3</b> (dpa = 2,2âČ-dipyridylamido),
have been studied using variable-temperature dc and ac magnetometry
and high-frequency EPR spectroscopy. All three compounds possess an <i>S</i> = 2 electronic ground state arising from the terminal
Cr<sup>2+</sup> ion, which exhibits slow magnetic relaxation under
an applied magnetic field, as evidenced by ac magnetic susceptibility
and magnetization measurements. The slow relaxation stems from the
existence of an easy-axis magnetic anisotropy, which is bolstered
by the axial symmetry of the compounds and has been quantified through
rigorous high-frequency EPR measurements. The magnitude of <i>D</i> in these compounds increases when heavier ions are substituted
into the trimetallic chain; thus <i>D</i> = â1.640,
â2.187, and â3.617 cm<sup>â1</sup> for Cr<sub>2</sub>CrÂ(dpa)<sub>4</sub>Cl<sub>2</sub>, Mo<sub>2</sub>CrÂ(dpa)<sub>4</sub>Cl<sub>2</sub>, and W<sub>2</sub>CrÂ(dpa)<sub>4</sub>Cl<sub>2</sub>, respectively. Additionally, the <i>D</i> value
measured for W<sub>2</sub>CrÂ(dpa)<sub>4</sub>Cl<sub>2</sub> is the
largest yet reported for a high-spin Cr<sup>2+</sup> system. While
earlier studies have demonstrated that ligands containing heavy atoms
can enhance magnetic anisotropy, this is the first report of this
phenomenon using heavy metal atoms as âligandsâ
Synthesis, Detailed Characterization, and Theoretical Understanding of Mononuclear Chromium(III)-Containing Polyoxotungstates [Cr<sup>III</sup>(HX<sup>V</sup>W<sub>7</sub>O<sub>28</sub>)<sub>2</sub>]<sup>13â</sup> (X = P, As) with Exceptionally Large Magnetic Anisotropy
Two monochromiumÂ(III)-containing
heteropolytungstates, [Cr<sup>III</sup>(HP<sup>V</sup>ÂW<sub>7</sub>O<sub>28</sub>)<sub>2</sub>]<sup>13â</sup> (<b>1a</b>) and [Cr<sup>III</sup>(HAs<sup>V</sup>ÂW<sub>7</sub>O<sub>28</sub>)<sub>2</sub>]<sup>13â</sup> (<b>2a</b>), were prepared
via simple, one-pot reactions in aqueous, basic medium, by reaction
of the composing elements, and then isolated as hydrated sodium salts,
Na<sub>13</sub>[Cr<sup>III</sup>(HP<sup>V</sup>ÂW<sub>7</sub>O<sub>28</sub>)<sub>2</sub>]·47H<sub>2</sub>O (<b>1</b>) and Na<sub>13</sub>[Cr<sup>III</sup>(HAs<sup>V</sup>ÂW<sub>7</sub>O<sub>28</sub>)<sub>2</sub>]·52H<sub>2</sub>O (<b>2</b>). Polyanions <b>1a</b> and <b>2a</b> comprise
an octahedrally coordinated Cr<sup>III</sup> ion, sandwiched by two
{PW<sub>7</sub>} or {AsW<sub>7</sub>} units. Both compounds <b>1</b> and <b>2</b> were fully characterized in the solid
state by single-crystal XRD, IR spectroscopy, thermogravimetric and
elemental analyses, magnetic susceptibility, and EPR measurements.
Magnetic studies on <b>1</b> and <b>2</b> demonstrated
that both compounds exhibit appreciable deviation from typical paramagnetic
behavior, and have a ground state S = <sup>3</sup>/<sub>2</sub>, as
expected for a Cr<sup>III</sup> ion, but with an exceptionally large
zero-field uniaxial anisotropy parameter (<i>D</i>). EPR
measurements on powder and single-crystal samples of <b>1</b> and <b>2</b> using 9.5, 34.5, and 239.2 GHz frequencies and
over 4â295 K temperature fully support the magnetization results
and show that <i>D</i> = +2.4 cm<sup>â1</sup>, the
largest and sign-assigned <i>D</i>-value so far reported
for an octahedral Cr<sup>III</sup>-containing, molecular compound.
Ligand field analysis of results from CASSCF and NEVPT2-correlated
electronic structure calculations on CrÂ(OH)<sub>6</sub><sup>3â</sup> model complexes allowed to unravel the crucial role of the second
coordination sphere of Cr<sup>III</sup> for the unusually large magnetic
anisotropy reflected by the experimental value of <i>D</i>. The newly developed theoretical modeling, combined with the synthetic
procedure for producing such unusual magnetic molecules in a well-defined
and essentially magnetically isolated environment, appears to be a
versatile new research area
Synthesis, Detailed Characterization, and Theoretical Understanding of Mononuclear Chromium(III)-Containing Polyoxotungstates [Cr<sup>III</sup>(HX<sup>V</sup>W<sub>7</sub>O<sub>28</sub>)<sub>2</sub>]<sup>13â</sup> (X = P, As) with Exceptionally Large Magnetic Anisotropy
Two monochromiumÂ(III)-containing
heteropolytungstates, [Cr<sup>III</sup>(HP<sup>V</sup>ÂW<sub>7</sub>O<sub>28</sub>)<sub>2</sub>]<sup>13â</sup> (<b>1a</b>) and [Cr<sup>III</sup>(HAs<sup>V</sup>ÂW<sub>7</sub>O<sub>28</sub>)<sub>2</sub>]<sup>13â</sup> (<b>2a</b>), were prepared
via simple, one-pot reactions in aqueous, basic medium, by reaction
of the composing elements, and then isolated as hydrated sodium salts,
Na<sub>13</sub>[Cr<sup>III</sup>(HP<sup>V</sup>ÂW<sub>7</sub>O<sub>28</sub>)<sub>2</sub>]·47H<sub>2</sub>O (<b>1</b>) and Na<sub>13</sub>[Cr<sup>III</sup>(HAs<sup>V</sup>ÂW<sub>7</sub>O<sub>28</sub>)<sub>2</sub>]·52H<sub>2</sub>O (<b>2</b>). Polyanions <b>1a</b> and <b>2a</b> comprise
an octahedrally coordinated Cr<sup>III</sup> ion, sandwiched by two
{PW<sub>7</sub>} or {AsW<sub>7</sub>} units. Both compounds <b>1</b> and <b>2</b> were fully characterized in the solid
state by single-crystal XRD, IR spectroscopy, thermogravimetric and
elemental analyses, magnetic susceptibility, and EPR measurements.
Magnetic studies on <b>1</b> and <b>2</b> demonstrated
that both compounds exhibit appreciable deviation from typical paramagnetic
behavior, and have a ground state S = <sup>3</sup>/<sub>2</sub>, as
expected for a Cr<sup>III</sup> ion, but with an exceptionally large
zero-field uniaxial anisotropy parameter (<i>D</i>). EPR
measurements on powder and single-crystal samples of <b>1</b> and <b>2</b> using 9.5, 34.5, and 239.2 GHz frequencies and
over 4â295 K temperature fully support the magnetization results
and show that <i>D</i> = +2.4 cm<sup>â1</sup>, the
largest and sign-assigned <i>D</i>-value so far reported
for an octahedral Cr<sup>III</sup>-containing, molecular compound.
Ligand field analysis of results from CASSCF and NEVPT2-correlated
electronic structure calculations on CrÂ(OH)<sub>6</sub><sup>3â</sup> model complexes allowed to unravel the crucial role of the second
coordination sphere of Cr<sup>III</sup> for the unusually large magnetic
anisotropy reflected by the experimental value of <i>D</i>. The newly developed theoretical modeling, combined with the synthetic
procedure for producing such unusual magnetic molecules in a well-defined
and essentially magnetically isolated environment, appears to be a
versatile new research area
Coherent Spin Dynamics in Molecular Cr<sub>8</sub>Zn Wheels
Controlling and understanding transitions
between molecular spin
states allows selection of the most suitable ones for qubit encoding.
Here we present a detailed investigation of single crystals of a polynuclear
Cr<sub>8</sub>Zn molecular wheel using 241 GHz electron paramagnetic
resonance (EPR) spectroscopy in high magnetic field. Continuous wave
spectra are well reproduced by spin Hamiltonian calculations, which
evidence that transitions in correspondence to a well-defined anticrossing
involve mixed states with different total spin. We studied, by means
of spin echo experiments, the temperature dependence of the dephasing
time (<i>T</i><sub>2</sub>) down to 1.35 K. These results
are reproduced by considering both hyperfine and intermolecular dipolar
interactions, evidencing that the dipolar contribution is completely
suppressed at the lowest temperature. Overall, these results shed
light on the effects of the decoherence mechanisms, whose understanding
is crucial to exploit chemically engineered molecular states as a
resource for quantum information processing